1. Field of the Invention
The present invention relates to an image forming system, such as a copying machine or a laser beam printer, and more particularly to an image forming system operable in dual modes, a normal print mode and a copy print mode.
2. Discussion of the Related Art
A conventional laser beam printer includes a plurality of sheet trays, for example, three sheet trays 1 to 3, as shown in FIG. 2. A sheet of paper 4 to be recorded thereon is selectively picked up from one of those trays 1 to 3 by means of a pick-up roller 5 when the roller receives a control signal from a host computer (not shown). The sheet 4 picked up by the pick-up roller 5 is transferred to paired registration rollers 8a and 8b as a sync transfer section by pairs of transfer rollers 6a and 6b and 7a and 7b. The paired regist rollers 8a and 8b are driven at a print start timing, that is, after a predetermined time elapses from the start of exposure of a photoreceptor drum 10 to light from an image forming section 9. The paired regist rollers 8a and 8b, when driven, brings the sheet 4 forward in time registration with the developed image on the photoreceptor 10. The image forming section 9 forms a latent electrostatic image on the photoreceptor 10. The latent image is developed by a developing section 11. The developed image is transferred to the sheet 4 by a transfer section 12. Thereafter, the image is thermally fused to the sheet 4 by means of paired fusing rollers 13a and 13b. The sheet 4 emanating from the fixing roller pair is transferred by paired transfer rollers 14a and 14b and paired exit rollers 15a and 15b, and is discharged to an output tray 16. However, in the laser beam printer, the angles of the sheets from the trays 1 to 3 to the paired regist rollers 8a and 8b are different from one another. The leading edges of the sheet 4, that are picked up from the trays 1 to 3 and transferred, reach the paired regist rollers 8a and 8b at different times. That is, the time taken for the sheet 4 to travel from the tray 2 to the regist roller pair is longer than that from the tray 1 to the roller pair; and the time taken for the sheet 4 to travel from the tray 3 to the roller pair is longer than that from the tray 2 to the roller pair. As a result, the through-put of the print processing (the amount of processing per unit time) is different for different trays. To cope with this, a timing chart as shown in FIGS. 11(a) through 11(g) has been used for the control of the operation from the sheet supply stage to the printing stage in the conventional printing system.
In this printing system, in response to a print command from the host computer, the base machine applies a transfer signal to the pick-up roller 5, which picks up and transfers the sheet 4. When a regist sensor 17 detects the arrival of the sheet 4, the base machine places the paired regist rollers 8a and 8b in an standby state and sends a vertical sync signal request signal to the host computer. Upon receipt of the request signal, the host computer sends a vertical sync signal and image data signal. In response to those signals, the base machine drives the paired regist rollers 8a and 8b to set up the time registration of the image on the photoreceptor 10 with the position of the sheet 4.
Thus, at instant that the sheet 4 from any one of the trays 1 to 3 arrives at the paired regist rollers 8a and 8b and that the base machine is ready for print operation, the base machine sends a vertical sync signal request signal to the host computer. At this time, if the image data to be printed has been prepared in the host computer, that is, the image data processing, such as conversion of the received character codes into image data and conversion of the coordinates of image data into portrait or land scape, has been completed, the host computer sends a vertical sync signal and the image data to the base machine.
In a normal print mode where image data is different for each print, the sheet 4 cannot be moved from the paired regist rollers 8a and 8b till the host computer produces a vertical sync signal, when the host computer produces a print command to the base machine, the sheet 4 arrives at the paired regist rollers 8a and 8b, and the base machine sends a vertical sync signal request signal to the host computer, but the image data processing, such as conversion of the received character codes into image data and conversion of the coordinates of image data into portrait and landscape, is not completed in the host computer.
In other words, the base machine can only enter the print operation if the host computer sends a vertical sync signal to the base machine when the host computer completes the above-described data processing. Therefore, in the conventional printing system thus constructed, it is unnecessary for the host computer to determine the timing of the print command depending on the amount of image data and the processing time. Particularly, in the case where the sheet 4 is picked up from the tray 3, transferred, and printed, the task of the host computer for processing the data to control the base machine, which is other than image data, is reduced, leading to improvement of the through-put.
In a copy print mode to print the same image data plural times, the conventional printing system needs, every print, the transfer of the vertical sync signal request signal and the vertical sync signal between the base machine and the host computer. The signal transfer operation is complicated and the processing task of the host computer is large.